4. Electric Dipole Moment and Molecular Structure 4.1 Electric dipole moment (電偶極子效率) Definition of electric dipole moment Electrical potential at position Electric charge Dipole moment Quadrupole moment + ・・・ 1 Dipole moment directs from – to + !!! Units of electric dipole moment Cm or D (Debye) 1D ≡ 1 cgs esu = 3.336 × 10-30 Cm Electric dipole moment and intermolecular interactions (分子間相互作用) Charge term ∝ Dipole term ∝ Quadruple term ∝ Long range force Short range force 2 Intermolecular interactions (分子間相互作用) Cf. Infrared electroabsorption spectroscopy Observed electroabsorption shift 0.01～0.1cm-1 Solvent shift 10cm-1 Solvent field is much higher than the external field !!! 3 Molecular symmetry and electric dipole moment Molecules with inversion symmetry (対称中心) inversion trans-1,2-dichloroethylene cis-1,2-dichloroethylene Cl H Cl Cl H Cl H H 無極性 non-polar 有極性 polar N-methylacetoamide O CH3 N H3C H O CH3 O CH3 N N H3C H H3C H 4.4 D 4.4 D × 2 monomer dimer A head-to-tail structure determined with infrared electroabsorption spectroscopy !! 4 4.2 Electric dipole moments of diatomic molecules a) NaCl (a high temperature molecule 高温分子) ← determined with microwave spectroscopy ← determined with microwave spectroscopy b) HCl 5 c) Ionicity of a bond （離子度） covalency（共價鍵度） iconicity（離子度） H:Cl H+Cl－ Neglect these cross terms HF 6.09 0.917 0.41 HCl 3.70 1.274 0.18 HBr 2.76 1.414 0.12 HI 1.49 1.609 0.057 Ionicity b2 decreases on going from HF to HI. HI is almost completely covalent !! 6 4.3 Electric dipole moments of polyatomic molecules and bond moments a) H2O bond moment of HO b) Bond moments (positive atom first like HO and C=N) HC Cm CN CO CS CI CBr CCl CF C=N C=O C=S C≡N CH3Cl 7 CHCl3 Cl-C-Cl angle is much larger than the tetrahedral angle !!! c) Dynamic polarization and fluctuating dipole moments（動態分極） in the gaseous phase MD simulation of acetone in acetonitrile O N C H D / 気相 双極子モーメント 時間 /ps 8 5. Dielectric Polarization and Dielectric Constant 5.1 Dielectric material and dielectric constant A capacitor in vacuo A capacitor with dielectric material 9 10 ：canal field ：gap field (Maxwell eq.) = 0E + P Total polarization Polarization: Dipole moment induced in a unit volume Dielectric constant ： proportional constant between and (linear polarization) 11 Relative permeability or dielectric constant Measurement of dielectric constant ：charge （電荷） C ：capacity （容量） ：potential difference （電位差、電壓）V = ED In vacuo With dielectric medium Resonant circuit to measure C ：inductance Measurement of with a resonant circuit with known L gives the value of C. 12 5.2 Lorentz field A field that is experienced by a molecule in a dielectric medium Electric field for –z direction that is produced by the charge induced on an area element Electric field for –z direction that is produced by the charge induced on the total sphere ← Lorentz field 13 Lorentz field is in between and !! 14 5.3 Molecular polarization（分子極化） Molecular polarization ：Electronic polarization （電子極化） ：Atomic polarization （原子極化） ：Orientational polarization （方向極化） Electronic polarization: polarization due to the distortion of the electron cloud Dipole moment induced on one molecule ：electronic polarizability A metal sphere in a uniform electric field ：number of molecule in a unit volume 15 Atomic polarization: polarization due to the distortion of molecular structure ：atomic polarizability Orientational polarization: polarization due to molecular orientation Only for polar molecules ：molecular dipole moment http://zh.wikipedia.org/wiki/%E4%BB%8B%E9%9B%BB%E8%B3%AA 16 -30 7 -1 -1 -23 -23 =10 Cm, E=10 Vm =NC → E=10 Nm=10 J -23 -1 -20 K=1,38x10 JK , T=300 K → kT=4x10 J 17 Molecular polarization and dielectric constant (Clausius-Mossoti formula) Macro Micro Temperature dependence of Determination of dipole moments !!! 18 4.4 Dielectric dispersion 4.5 Non-linear polarization Linear Non-linear Vibration Linear optics ← Light propagation, emission, absorption Non-linear optics ← Sum and difference frequency generation: 2 19 CARS Coherent Anti-Stokes Raman Scattering CARS 20 .